This invention arose out of a need to control the power take-off shaft on a farm tractor. However the disclosure is not to be limited to this environment as the two stage pressure control valve is adapted for many other uses, such as any use where a motive force is to be discontinued for a period of time and during this time another function is to be activated.
It is often important to control the rate at which the power take-off shaft on a farm tractor is engaged and disengaged because machinery that is driven by the power take-off shaft has great mass. Accelerating and decelerating the mass without gradual control of the various components is essential. In the past, the farmer needed to have experience in timing the engagement of the power take-off shaft as well as the disengagement and braking thereof. This manual control often times resulted in the controlled machinery being decelerated too rapidly causing component failure on the machinery or resulted in rapid acceleration also causing component failure. Accordingly, it is a desire to provide a mechanism for controlling the engagement as well as the disengagement and braking of the power take-off shaft in a fully automated manner.
Accordingly, it is an object of the invention to provide a pressure control valve having two control ports, wherein the first control port is initially at a high pressure while the second control port is at a low pressure and thence effecting a systematic control so as to cause the first control port to be changed to a lower pressure condition thereafter followed by an increase of the pressure at the second control port to a high pressure and vice versa.
It is a further object of the invention to provide a reciprocal spool in the pressure control valve, the spool having a pair of opposing control surfaces such that pressure in the first and second control ports is systematically applied to the control surfaces so as to control the position of the spool to improve valve performance, such as repeatability of control functions and pressure sensitivity.
It is a further object of the invention to provide a pressure control valve that is manufactured as a unit and is capable of being installed as a unit into a valve housing without any alteration of the piping leading to the valve housing.
It is a further object of the invention to provide a pressure control valve having two control ports, with each control port being controlled by a separate spool reciprocally mounted in the valve body.
It is a further object of the invention to provide a pressure control valve, as aforesaid, which is controlled by a first stage pilot which has a single solenoid coil and produces a pressure output at a pilot outlet port that is proportional to the electrical signal value applied thereto.
It is a further object of the invention to provide a pressure control valve, as aforesaid, wherein movement of the spool caused by the pilot pressure at the pilot outlet port is resisted by a bias spring, and wherein pressure in the respective control ports is systematically applied to the spools in opposite directions so that, in conjunction with the spring load, the spool is balanced in every position against an equivalent pilot load pressure at the pilot outlet port.